Increased concern for the prevention of the release of gaseous by-products of the kraft pulping process has focused attention upon the control of gaseous emissions. The kraft process produces such gases as hydrogen sulfide (H.sub.2 S), methyl mercaptan (CH.sub.3 SH), dimethyl sulfide (CH.sub.3 SCH.sub.3) and dimethyl disulfide (CH.sub.3 SSCH.sub.e)--collectively known as Total Reduced Sulfur (TRS) gases. These gases are corrosive, toxic and explosive under certain conditions. Care must be taken in their collection, transport and destruction. In a kraft pulp mill, the collection, transportation and destruction system that treats these gases is known as the non-condensible gas (NCG) system.
One source of low-concentration, high-volume NCG's is the chip bin of a continuous digester. Chips fed to a continuous digester are typically first exposed to steam to remove air. In a typical Kamyr.RTM. atmospheric chip steaming system, as sold by Kamyr, Inc. of Glens Falls, N.Y., fresh steam and steam produced in the flashing of black liquor are used as a source of steam in both the steaming vessel and chip bin. Since some of this steam is produced from flashed black liquor it typically contains TRS gases.
Ideally, the steam introduced at the base of the chip bin, or which leaks past the low pressure feeder below, rises in the chip column and gradually cools and condenses within the chip column. However, under certain conditions the steam does not condense but passes through the chip column and collects in the enclosed space above the column. Due to the potential corrosivity, toxicity and flammability of these gases they must be removed and, typically, condensed and incinerated, using a typical NCG gas outlet to the condenser. The gases are typically drawn to the condenser by a fan. Also, the presence or absence of these gases above the chips causes the pressure within the bin to fluctuate. Therefore, means must also be provided to prevent the over pressurization of the bin or collapse due to vacuum.
The chip bin typically includes steam introduction piping, an oscillating discharge, a gamma radiation level control, and a temperature probe. The temperature probe is an RTD-type averaging temperature sensor that is rigidly suspended approximately along the centerline of the chip bin. Conventionally, the temperature is only sensed and averaged along a five-foot length at the end of the probe. The remainder of the probe length is for wiring and support. The location of this sensing section varies from bin to bin and its location is dependent upon the production rate and retention time required.
The temperature measured by the temperature sensing probe sensing portion is used to control the addition of steam to the bin, by controlling a multiple position (e.g. infinitely variable) valve. Typically, the temperature measured by the probe is controlled to around 180.degree.-190.degree. F. (82.degree.-88.degree. C.). If the temperature falls below the setpoint, more steam is added; if above, steam flow is reduced.
A vacuum and pressure relief outlet is provided in the vent for NCG. This outlet typically includes the relief device disclosed in U.S. Pat. No. 5,169,498. This device uses a gate which allows for the controlled relief of overpressure and vacuum conditions to prevent damage to the bin. Though this gate is an effective relief device, under some conditions the design of this gate permits gross variation in gas flow through the gate. The bin usually operates under a vacuum condition, typically 6" of water vacuum, due to the draw of the NCG exhaust fan. The counterweights on the '498 patent gate are typically set so that at 6" of vacuum the gate is stationary in the closed, vertical position. However, deviation more or less from the vertical can result in a large increase in flow area about the gate. This may result in undesirable gross variations in the gas flow to the NCG system.
According to the present invention a method of steaming wood chips in a chip bin is provided, as well as a chip bin construction, that overcome the problems discussed above. In particular according to the present invention it is possible to substantially prevent cool, non-condensible gas-laden chips from accumulating at the top of the chip column, which can overload the NCG system, as well as preventing steam blow through (in which steam passes completely through the chip column into the NCG system). According to the invention it is also possible to provide a vacuum pressure relief device which minimizes the potential for gross gas flow variations thereacross as compared to conventional systems such as shown in U.S. Pat. No. 5,169,498.
According to a first aspect of the present invention, the steam addition to the chip bin is controlled as a function of both chip temperature and chip level. To practice this aspect of the invention a temperature probe is utilized with a longer averaging temperature sensing area. The temperature sensing portion of the probe extends across the working level of the chip column. This longer sensing area permits a more accurate measurement of the average chip pile temperature as the level varies. Conventional chip levels may vary by ten to fifteen feet. Ideally, it is desired that steam condense about five feet below the surface of the chip pile, thus the sensing area of the probe need only be long enough to sense to five feet below the surface. However, the sensing area of the temperature probe may be as long as the typical level variation of the chips, i.e., ten to fifteen feet.
According to one aspect of the present invention a method of steaming comminuted cellulose material in a chip bin having a top and bottom, a cellulose material inlet at the top, material outlet at the bottom, an interior temperature sensor, a non-condensible gas vent from the top, and a material level sensor, is provided. The method comprises the steps off (a) Feeding comminuted cellulose material into the material inlet to establish a column of material in the chip bin, having a level below the top of the chip bin. (b) Withdrawing steamed cellulose material from the material outlet, from the bottom of the column of material established in the chip bin. (c) Sensing the interior temperature of the chip bin across the level of the material, and the level of cellulose material column within the chip bin. (d) Venting non-condensible gases through the non-condensible gas vent. (e) Feeding steam to the chip bin below the level of the cellulose material column to steam cellulose material in the column. And, (f) controlling the feeding of steam in step (e) dependent upon both the sensed interior temperature and level of material so as to substantially prevent cool, non-condensible gas-laden chips from accumulating at the top of the column and to substantially prevent steam blow-through.
Step (f) is typically practiced by utilizing a formula which takes into account both the temperature and the level of the chips. While a number of different formulas can be used based upon different assumptions, and depending upon different requirements, a particular formula that is useful according to the invention is: ##EQU1## Where
Tsp is the temperature setpoint for steam addition, in degrees Celsius;
Tv is the desired temperature of non-condensible gases in the vent, and is specified as some increment over ambient temperature, in degrees Celsius;
L is the chip bin level in percent (%); and
K is an arbitrary calibration constant with a typical value between 0.8 and 0.99, which prevents control instabilities should the level, L, reach 100%,
so as to add steam whenever the temperature sensed in step (c) is below Tsp.
Step (f) may be further practiced so that Tv is about 5.degree.-20.degree. C. above ambient temperature, the increment over ambient temperature utilized typically varying in dependence upon the species of cellulose material added in step (a), the season of the year, and other factors.
Step (c) is typically practiced utilizing a temperature sensing probe extending from the top of the chip bin into the material column and having a temperature sensing portion length of about ten-twenty (preferably ten to fifteen) feet, and having a bottommost portion that extends about five feet below the expected level of material in the column.
According to another aspect of the present invention a chip bin is provided which comprises the following elements: A generally vertical vessel having a top and a bottom. A cellulose material inlet at the top for feeding cellulose material into the vessel to establish a column of comminuted cellulose material therein, the column having a level. A material outlet at the bottom. An interior temperature sensor comprising a probe extending from the vessel top into the vessel and across the cellulose material column level, and having a temperature sensing portion. A non-condensible gas vent from the vessel top. A material level sensor. Means for feeding steam into the vessel below the level of the cellulose material column to steam cellulose material in the column. And, means for controlling the steam feeding means dependent upon both the sensed temperature from the interior temperature sensor and the sensed level of material from the material level sensor so as to substantially prevent cool, non-condensible gas-laden chips from accumulating at the top of the column and to substantially prevent steam blow-through.
The temperature probe temperature sensing portion typically has a length of at least ten feet, e.g., about ten-twenty feet, and may be an RTD-type. The material level sensor typically comprises, as is conventional, a gamma detector. The steam feeding controlling means typically comprises one or more steam conduits each having a valve therein. The valves are typically infinitely variable position valves, and at least have multiple open positions and a closed position. A controller also controls the position of each of the valves as part of the steam feeding controlling means.
According to the invention a vacuum and pressure relief device is also preferably disposed in a non-condensible gases vent. According to the present invention the vacuum and pressure relief device comprises means for minimizing the potential for gross gas flow variations thereacross, compared to the device such as shown in U.S. Pat. No. 5,169,498.
The vacuum and pressure relief device according to the invention typically comprises the following components: A solid interior peripheral portion of the vent. A gate mounted for pivotal movement with respect to the solid interior peripheral portion by a pivot shaft at a first end of the gate, the gate having a second, free, end opposite the first end. And a casing comprising a sector of a cylinder and having a closed curved exterior surface and closed first and second ends and open first and second sides, the cylindrical sector casing mounted in the solid interior peripheral portion so that the open sides thereof communicate with the vent and so that the casing surrounds the gate between the first and second ends thereof, the gate pivotal about the pivot shaft with respect to the casing.
Also a brush or similar type seal is preferably provided between the gate and at least the closed first and second ends of the cylindrical sector casing (the seal may also be provided along the closed curved interior surface of the casing). The seal may be mounted on the casing or the gate or both. Also a counterweight is preferably mounted on the pivot shaft, generally in the same manner as shown in U.S. Pat. No. 5,169,498 (the disclosure of which is hereby incorporated by reference herein).
The cylindrical sector has a sector angle of about 30.degree.-60.degree. so that the curved exterior surface also extends about 30.degree.-60.degree., preferably about 45.degree.. By providing the casing covering this degree of arc, rather than merely providing a narrow closure area for the gate such as shown in U.S. Pat. No. 5,169,498, gross variations in the gas flow to the NCG system may be minimized.
It is the primary object of the present invention to provide a chip bin, and method of steaming chips in the chip bin, which prevents overload of an NCG system so as to minimize the emission of TRS gases. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.